A series of contemporary studies [1]}, [2]} suggest that the ringdown signal may provide a conclusive proof for the formation of an event horizon. Such expectation is based on the assumption that an ECO would possess a reflective surface beyond the PS instead of an event horizon. This would lead to the existence of a trapping region between the surface of the ECO and PS where perturbations could be confined and manifest themselves as echoes in the late stage of the gravitational waveform. Then, by considering ringdown waveforms of ECOs, such as wormholes, it has been claimed [1]}, [2]} that precision observations of the late-time ringdown signal can distinguish between the formation of ECOs or BHs.
